They say in the article that the conversion efficiency is 90%, which I assume means that if the reagent supply is infinite, 90% of the electrical energy you put into the catalyst is turned into chemical bond energy.
It says the Faradaic efficiency is 90%, which unfortunately is not the total energy efficiency. Total energy efficiency also includes thermodynamic and practical constraints.
https://en.wikipedia.org/wiki/Faraday_efficiency#Faradaic_lo...
However that's still a great figure.
I'm in awe that a carbon-substrate catalyst converts CO2 so well without degrading itself.